DFT SCF Core
This page covers the legacy DFTSystem/run_scf core. It stays intentionally
small: spin-unpolarized Γ-point SCF, orthorhombic cells, local Gaussian toy
pseudopotentials, and ion-backed GTH/UPF pseudopotential prototypes. It is not
the periodic PeriodicDFTSystem/run_periodic_scf materials path.
Exchange-Correlation
Section titled “Exchange-Correlation”DFT does not solve directly for the full many-electron wavefunction. In the
Kohn-Sham construction, auxiliary orbitals generate the density ρ(r), and the
unknown many-electron physics is approximated by an exchange-correlation
functional.
This milestone exposes that as a small API:
DiracExchangeLDACorrelationPZ81LDAExchangeCorrelationPBEExchangeCorrelation
Each functional returns an energy density, total energy, and potential v_xc.
The default SCF path uses LDA exchange plus PZ81 correlation.
PBEExchangeCorrelation is public alpha API: it uses the PBE GGA gradient form
with the package’s PZ81 uniform correlation baseline, so it reports itself as a
PBE-PZ81 alpha diagnostic rather than full PW92-backed production PBE.
SCF Mixing
Section titled “SCF Mixing”SCF iteration repeatedly maps an input density to a new output density. Directly replacing the old density is often unstable, so the code now has two mixers:
LinearMixer: conservative and predictable.PulayDIISMixer: uses residual history to accelerate convergence.
SCFResult records density residual, potential residual, energy delta,
convergence status, failure reason, timing totals, and mixer metadata.
Restartable State
Section titled “Restartable State”run_scf(...) accepts initial_density and initial_orbitals. This is enough
for continuation workflows and future geometry steps. A restart density is
renormalized to the requested electron count so continuation does not silently
change the number of electrons.
Forces
Section titled “Forces”local_pseudopotential_forces(...) computes Hellmann-Feynman-style forces on
Gaussian centers from a converged or fixed density. Ion-backed systems add
center-center forces, and systems that apply nonlocal projectors report a
fixed-orbital finite-difference nonlocal correction. These are diagnostic alpha
forces, not production DFT force claims.
Performance Evidence
Section titled “Performance Evidence”The DFT benchmark now reports per-case timings for Hartree solves, XC evaluation, kinetic application, mixer cost, force evaluation, total SCF time, and a compact FFT probe. This makes the next optimization decision evidence based instead of speculative.